Ocean acidification may be the “topic du jour” in climate science, but most of the existing research still focuses on how calcifying organisms, like corals and shellfish, are impacted. With ocean acidification and other climate-linked stressors likely to impact all marine organisms, researchers are beginning to delve more into effects on non-calcifying species and their communities.

Atmospheric carbon dioxide (CO2) dissolves in the water column through a series of breakdowns and interactions and effectively lowers the pH, causing acidification. At the same time, atmospheric CO2 is helping to trap heat from the sun, resulting in the warming of both terrestrial and marine environments. These two environmental factors are influencing ecosystems around the world.

Neosiphonia harveyi

Neosiphonia harveyi is an invasive species of red algae native to Asia that was introduced into the Atlantic in the 20th century. Neosiphonia is one of the most widespread species and can now be found in both the eastern and western Atlantic.

Seaweeds like Neosiphonia play an important role in marine ecosystems. They are primary producers, using photosynthesis to convert the sun’s energy into sugars, which in turn provide energy for the rest of the ecosystem. While excess CO2 is proving detrimental to calcifying organisms, the story for seaweeds is different because CO2 is a necessary molecule for photosynthesis.

The Study:

This study looks at the effect of increased CO2 and temperature on the growth and productivity of the widespread red algae, Neosiphonia harveyi. Researchers took individual specimens of Neosiphonia harveyi and subjected them to 3 levels of CO2; one at present atmospheric CO2 level and the other two representing projected future conditions. The specimens were also treated with one of two temperatures, 10°C and 17.5°C. The researchers had 2 goals; the first goal was to determine how temperature and CO2 interact to affect the growth and productivity of the algae. The second was to investigate the role of naturally occurring carbon-concentrating mechanisms (CCMs), a protein-based mechanism for assimilating carbon from the surrounding environment, under increased levels of CO2.

Using stock-cultured algal individuals, researchers exposed organisms to the different experimental treatments within a controlled laboratory setting. After 2 weeks of treatment, photosynthetic activity, relative growth rates, and chlorophyll fluorescence (a measurement of energy released by a primary producer, used as a metric for physiological health and stress) were measured.

The results of this study are quite interesting and the researchers were able to make some inferences about the future spread and success of invasive species. It was shown that increases in CO2 lead to increases in photosynthetic activity (Figure 1), chlorophyll fluorescence, and overall growth of the individual (Figure 2). These increases were documented at each temperature, meaning CO2 supported productivity and growth both alone and interactively with temperature. It was also found that the beneficial effects of increased CO2 were greater with lower temperature. Researchers also investigated the presence and ultimately the function of the carbon-concentrating mechanism in Neosiphonia. After determining the presence of a CCM, researchers used inhibitors to block the ability of the CCM and found that photosynthesis was inhibited at all temperatures and CO2 levels.

Figure 1: The net-photosynthesis of treated algae under different levels of carbon dioxide and temperature. Letters above the bars represent significant differences determined by statistical analysis.

Figure 2: The relative growth rates (RGR) in percent per day in respect to the treatments. Letters above the bars represent significant differences determined by statistical analysis.

Significance

Arguably the most significant finding from this study is that Neosiphonia saw greater increases in growth and productivity at lower temperatures than at higher temperatures. Growth and productivity was relatively low at normal CO2 levels and at low temperatures; this is typical as many species of algae are not cold tolerant. However, when CO2 was increased under the cold treatment, algae responded with a significant increase in growth and photosynthetic activity. It seems as though elevated CO2 can relieve effects of low temperature on this species of algae.

Corals and algae compete for space and resources in a reef.

Marine invasive species are often range limited by temperature. If the results of this study apply to other organisms, it would appear that elevated CO2 could aid the growth, spread, and success of invasive species. While this study highlights the potential success of algae, this success comes at a cost to the ecosystem. As humans continue to alter the climate, unique habitat providers (such as corals and kelps) are projected to be overtaken by fleshy, turf-forming algae. As these species are replaced, the diversity and function of the system is likely to decrease. If the changing climate also aids the spread of species like Neosiphonia, it could accelerate the loss of community diversity and help create a sea full of weeds.

I am currently a postdoc at Keck Sciences, Claremont McKenna College. I work with Dr. Sarah Gilman, measuring and modeling energy budgets in intertidal species. I am a climate scientist and marine community ecologist and my PhD (University of Rhode Island) focused on how ocean acidification and eutrophication, alters coastal trophic interactions and species assemblages.